Method and arrangement for digital transmission using am emitters

ABSTRACT

During digital transmission using existing non-linear AM emitters, spurious emissions occur as a result of the non-linear distortions, in turn causing inner band disturbances and also disturbing adjacent channels, as out-of-band radiation. Non-linear distortions are especially critical for digital multiple carrier signals (e.g., OFDM) which are recommended by the ITU with the DRM system for the AM area. In order to avoid the non-linear distortions, the end step of the AM emitter is operated in the linear mode, thus ensuring the ITU spectrum mask. The relatively low efficiency of the emitter during the linear operation can be improved by tracking the distribution voltage of the emitter end step according to the drive. To this end, the envelope of the complex modulated data signal is scanned, and said signal controls the distribution voltage for the emitter end step by means of the modulator operating as a switched-mode power supply unit.

[0001] The present invention relates to the field of broadcast transmitters which will be converted from analog amplitude modulation (AM) to digital modulation as digitalization moves forward.

[0002] In this context, the intention is for the hitherto usual transmitter types, non-linear AM transmitters featuring an RF input (radio frequency) and an audio input, to continue in use. The reasons for this are as follows:

[0003] AM transmitters internally operate in switched mode and therefore have efficiencies which are better by a factor of 3 than those of linear transmitters which are otherwise usually used for digital transmission, for example, in the case of DAB (Digital Audio Broadcasting) and DVB (Digital Video Broadcasting). This results in a saving of operating costs.

[0004] it is easier to convince broadcasters to convert from analog to digital if no great investments come up in the preliminary stages.

[0005] The digitalization of AM broadcasting is seen as the only chance to preserve these frequency ranges and the technology used therein in the long term. For implementation purposes, the consortium “Digital Radio Mondiale” was founded, see “Rundfunktechnische Mitteilungen” [Broadcasting Newsletter], 43rd year, 1999, issue 1, pages 29-35.

[0006] The use of a non-linear AM transmitter for digital modulation requires a special operating mode of the transmitter. The modulated digital signal is generated by two partial signals (I and Q), which are orthogonal to each other. The I-signal (“in phase”) is modulated onto a cosine oscillation having the frequency Ft (carrier frequency). The Q-signal (“quadrature”) is modulated onto a sine oscillation having the same frequency Ft. The sum of both modulated oscillations produces the complex modulated data signal (cosine 0 180 degrees, sine 90-+90 degrees). The modulated I/Q-signal is shaped by filters in such a manner that it has exactly the prescribed curve shape with the desired bandwidth.

[0007] For non-linear operation, it is required for the modulated I/Q-signal to be converted in such a manner that two signals, an amplitude signal (A-signal) and a phase-modulated carrier signal (RF-P), result therefrom that are suitable for proper control of the AM transmitter. Then, at the output of the AM transmitter, the modulated I/Q-signal is generated again with higher power.

[0008] The modulated I/Q-signal corresponds to a Cartesian representation. The Cartesian representation is converted to a polar representation with amplitude and phase. In this manner, the amplitude signal (A-signal) is obtained to control the AM transmitter at the audio input. A phase-modulated radio frequency (RF-P signal) is generated from the initially resulting phase signal (P-signal). Advantageously, the RF-P signal can also be directly obtained without the intermediate step via the P-signal. In this manner, the signals are obtained that are required for controlling the AM transmitter:

[0009] amplitude signal (A-signal)

[0010] phase-modulated RF signal (RF-P signal)

[0011] The A-signal is fed into the modulator input (audio input) of the AM transmitter, and the RF-P signal is used for HF-type control of the transmitter. In the transmitter output stage, the two signals A&RF-P are multiplicatively combined, forming the high frequency digital output signal.

[0012] Due to the required conditioning process, both the A-signal and the RF-P signal obtain far larger bandwidths than the one the digital signal originally had and is intended to have again at the output of the transmitter.

[0013] Older modulators are frequently not able to provide the increased bandwidths (factor 3-5) because they were not designed for this. When using only the limited bandwidth that “older” transmitters have available in the modulator section, then this results in considerable out-of-band and spurious emissions. These have the property that they have only a very small gradient in the spectrum and therefore interfere with quite a number of adjacent channels.

[0014] Moreover, the spurious emissions generally lie above the limits that are coordinated by the ITU so that approval appears to be uncertain.

[0015] Non-linear distortions are particularly problematic when the intention is to transmit multicarrier signals, for example, OFDM (Orthogonal Frequency Division Multiplexing) signals, as digital modulation.

[0016] In the case of the DRM system (Digital Radio Mondiale) for digital transmission in the AM bands, which is currently recommended by the ITU for standardization, an OFDM technique using approximately 200 carriers is proposed as multicarrier technique.

[0017] Multicarrier modulations indeed have a nearly rectangular spectrum but feature a noise-like character in the time domain, namely both for the I-component and for the Q-component of the time signal. This is a result of the superposition of many statistically virtually independent subchannels that occurs in the process. According to the rules of the “Central Limit Theorem”, such a superposition has a distribution density function of the amplitude values, both of the I-component and of the Q-component, which nearly reaches the shape of a Gaussian bell-shaped curve. In such a case, the distribution density function of the amplitude values of the composite signal has the shape of a Rayleigh distribution. This means that small and medium amplitude values occur quite frequently whereas high amplitude values occur very rarely.

[0018] If the amplitude signal of an AM transmitter which is operated in this non-linear mode is amplitude-limited, then non-linear distortions occur which, on one hand, result in increased out-of-band and spurious emissions and, on the other hand, also cause inband interference which can be considerably higher than the out-of-band and spurious emissions due to the operating mode of the transmitter. The inband interference reduces the attainable coverage area since an already inherently noisy signal can tolerate less disturbances in the radio channel to get to a critical threshold at the receiver.

[0019] The present invention describes a method and arrangement for digital transmission using conventional AM transmitters by which unwanted emissions due to non-linear distortions are avoided to the greatest possible extent.

[0020] Non-linear distortions can be prevented if the operating point of the transmitter is shifted such that a linear mode of operation arises. For linear operation, the transmitter output stage is driven by the complex modulated data signal (I/Q-signal), as is known from the digital systems DAB and DVB.

[0021] The linear operation of the transmitter is advantageous with respect to the spurious emissions. These have spectrally much larger gradients than in the previously described non-linear mode which will allow compliance with the ITU spectrum mask combined with a good alignment of the transmitter. Only the efficiency of the transmitter is very low in linear operation, causing high costs of electricity.

[0022] The efficiency during linear operation of the AM transmitter is so poor because the full supply voltage is applied to the transmitter output stage even when the drive of this stage is low, and because power is converted to heat due to the quiescent current of the transmitter output stage. An improved efficiency can be achieved by making the supply voltage not much larger than required by the instantaneous drive of the output stage.

[0023] To correct the supply voltage for the transmitter output stage as a function of the instantaneous drive, the envelope of the complex modulated data signal is scanned by an amplitude detector (envelope rectifier or peak rectifier) and the supply voltage or anode voltage of the output stage is controlled by the modulator, which operates as a switched-mode power supply unit.

[0024] It is particularly important that no overdriving occur within the framework of the correction, not even for a short period of time. Overdrive could occur in that the envelope of the digital signal increases faster than is achieved by the correction of the supply voltage. As a rule, this assumption has to be made since the modulator just does not have the required bandwidth. This disadvantage can be eliminated in that the complex digital signal, subsequent to scanning its envelope, is delayed in a delay stage in such a manner that the supply voltage of the transmitter output stage can be corrected in the meantime. The amplitude detector and the delay stage have to be retrofitted into the transmitter on the occasion of the conversion to digital operation (see FIG. 1).

[0025] The time constant of the envelope detector must be such that it is possible to immediately follow a rise in the envelope so that no overdriving occurs with the distortions and spurious emissions resulting therefrom. However, other than is usual, for example, with “dynamic amplitude modulation”, the time constant for the decay can be selected to be exactly as large as for the rise because here it is not required to consider the “auditory impression”. The smaller decay time constant increases the efficiency of the transmitter further.

[0026] Transmitters which operate with pulse duration modulation (PDM) or with pulse step modulation (PSM) have such modulators in the form of switched-mode power supply units. The voltage obtained from the scanned envelope of the digital signal is used for controlling these PDM or PSM modulators, thereby exactly achieving the correction of the supply voltage for the transmitter output stage according to the envelope of the digital signal. Thus, both objectives are achieved: linear operation and increase of the efficiency of the transmitter to an acceptable value.

List of Reference Numerals Used

[0027]1 Amplitude detector for scanning the envelope

[0028]2 Delay stage for the complex modulated data signal

[0029]3 High-frequency preamplifier stages

[0030]4 Transmitter output stage

[0031]5 Driver stage of the modulator for correcting the supply voltage

[0032]6 Power stage of the modulator for correcting the supply voltage

[0033]7 Smoothing low pass of the modulator

[0034]8 Output filter of the AM transmitter 

1. A method for digital transmission using AM transmitters in which, due to the non-linear operating mode during digital transmission, non-linear distortions arise which result in inband interference and the out-of-band and spurious emissions, wherein the output stage of the AM transmitter is operated in the linear mode; the linear mode is operated in conjunction with a correction of the supply voltage of the transmitter output stage as a function of the instantaneous drive in order to improve the efficiency; the modulator of the AM transmitter operates as a switched-mode power supply unit and delivers the corrected supply voltage for the transmitter output stage; the envelope of the complex modulated data signal is scanned, and this signal controls the correction of the supply voltage for the transmitter output stage; the time constant during the scanning of the envelope is such that it is possible to immediately follow a rise in the envelope; the time constant during the scanning of the envelope can be equal for the rise and decay of the envelope; and the complex modulated data signal, subsequent to scanning its envelope, is delayed in such a manner that the correction of the supply voltage is effective in the meantime, thus preventing even short-duration overdrives of the transmitter output stage.
 2. The method as recited in claim 1, wherein the modulator operating as a switched-mode power supply unit can also be a pulse duration modulator or a pulse step modulator; and in the case of AM transmitters with class B push-pull modulators, a replacement with one of these modulators has to be carried out.
 3. An arrangement for digital transmission using AM transmitters in which the transmitter output stage is operated in the linear mode to avoid non-linear distortions and whose supply voltage is corrected by the complex modulated data signal as a function of the drive in order to improve the efficiency, wherein an amplitude detector (1) which scans the envelope of the complex modulated data signal is connected upstream of the modulator (5 and 6) operating as a switched-mode power supply unit; and a delay stage (2) for the complex modulated data signal is installed upstream of the high-frequency preamplifier stages (3) in the signal path to the transmitter output stage (4). 